Alkali metal silicate-zinc hydroxychloride coating composition

ABSTRACT

An aqueous, alkaline, inorganic composition, comprising water-soluble alkali metal silicate and zinc hydroxychloride, is used to coat substrates normally infested by algae, such as silicate-containing building material, e.g., concrete roofing tiles, to prevent or minimize algal infestation on said substrates, said algae otherwise imparting a dark, unsightly appearance thereto.

This is a division of application Ser. No. 358,030, filed May 7, 1973.

BACKGROUND OF THE INVENTION

This invention relates to a composition useful in coating substratesnormally infested by algae, such as silicate-containing buildingmaterial, e.g., concrete roofing tiles, to render said substrateresistant to algal infestation.

For many years there has been a so-called fungus or mildew problemassociated with the concrete tiles used on roofs of dwellings in thecoastal areas of Florida. The problem manifests itself as a severediscoloration of the roofs, creating an unsightly appearance thereof.The dominant microorganism causing the problem is not fungus but ratherGloeocapsa magma, a blue-green alga, although other strains of algae andsometimes fungi are found. The algae produce irregular, dark blotches inabout 1/2 to 2 years, the blotches continually increasing in area,generally as dark streaks on sloping roofs, and, if not remedied, thealgal infestation eventually results in discoloration of the entireroof. A new roof of concrete tiles can change from a pleasing whiteappearance to an unsightly black appearance, e.g. in 1 to 5 years, ifremedial steps are not taken. In addition to being unsightly,dark-colored roofs absorb large amounts of solar energy, such roofsbeing particularly undesirable in warm, sunny climates such as the GulfCoast.

Present practice to remedy the discoloration of algae infested roofsentails cleaning and painting them on a regular basis. Commonly, theroof is first treated with a strong algicide, e.g. sodium hypochlorite,to kill the algae and steam or a high pressure water spray is then usedto remove the algae residue. Then an organic coating, such as an acrylicor vinyl paint, often blended with Portland cement, whiting, or otherfiller, is applied. The organic paints are usually ready hosts for manytypes of microorganisms and do not prevent the renewed discoloration bythe microorganisms; the discoloration normally returns in about 6 to 18months, necessitating further cleaning and repainting. The discolorationis aggravated because organic-based paints tend to yellow with age anddeteriorate upon exposure to ultraviolet radiation. Numerous organiccompounds have been added to the paints in an attempt to control orprevent the growth of algae on roofs. These compounds generally do notprovide long-lasting effects because of their poor weather resistance,and discoloration occurs nearly as soon as if the compounds were notused. Many of the compounds used for this purpose contain mercury andthe use of mercury compounds is being restricted for ecological reasons.

Algicidal roofing granules coated with a fired inorganic alkali metalsilicate containing zinc, zinc oxide, or zinc sulfide are known (seeU.S. Pat. No. 3,507,676). Roofing granules coated with zinc oxychloridecontaining a coloring material are also known (see U.S. Pat. No.2,091,991), the zinc compound being used to obtain good bonding actionbetween the base granules and the coloring material.

Coating of substrates, such as concrete, with an anhydrous mixture ofzinc chloride dissolved in a volatile anhydrous solvent and zinc oxideto form a zinc oxychloride coating is also known (see U.S. Pat. No.2,310,128). Zinc oxychloride has also been used to coat zinc oxidepigment used in organic coatings, linoleum, rubber, textiles, etc., inan effort to inhibit the growth of fungus, mold, mildew, and the like(see U.S. Pat. No. 2,769,716).

Briefly, the aqueous, alkaline, inorganic coating composition of thisinvention comprises a solution of water-soluble alkali metal silicatewith zinc hydroxychloride dispersed in said solution. The composition isapplied to a substrate, e.g., by brushing, and allowed to dry undernormal ambient temperatures, thereby forming a hard, durable,weather-resistant, water permeable, substantially water insolublecoating on the substrate that renders the substrate resistant to algalinfestation which otherwise would impart a dark, unsightly appearancethereto.

The water-soluble alkali metal silicates used in this invention arewell-known materials and have been used as coatings for roofing granules(e.g., see U.S. Pat. No. 2,379,358). Though any of these silicates canbe used herein, the preferred alkali metal silicate in the practice ofthis invention is sodium silicate because of its relatively low cost andready availability, and this invention will be exemplified by thatsilicate.

The sodium silicate is used in preparing the coating composition of thisinvention in the form of an aqueous solution thereof. The concentrationof the silicate solution can vary and generally will be dictated by theviscosity desired in the coating composition after the zinchydroxychloride is mixed with the silicate solution, that viscositybeing such as to facilitate the mode of application of the coatingcomposition to the desired substrate. For example, where the zinchydroxychloride is added to the silicate solution in the form of anaqueous paste (containing, for example, 45 to 65 weight percent water)and the resultant coating composition is to be applied to the substrateby brushing, the sodium silicate solution used in making up such acoating composition will generally contain about 65 to 75 weight percentwater.

Sodium silicates useful in the practice of this invention generally willhave a weight ratio of Na₂ O:SiO₂ of 1:2.75 to 1:3.75. Aqueous sodiumsilicate solutions which are commercially available and can be used inmaking up the coating composition of this invention include those soldunder the trademarks "PD", "K", "N", "O", and "S-35", which areobtainable from the Philadelphia Quartz Company. Generally, thesecommercial products contain about 60 weight percent water and may haveto be diluted when used in preparing the coating compositions of thisinvention to provide the aforementioned concentration. Furtherinformation on the nature of alkali metal silicates useful in thepractice of this invention will be omitted in the interest of brevity,reference being made to "Soluble Silicates: Their Properties and Uses,"J. G. Vail, Reinhold Publishing Co., New York (1952), for a furtherdescription thereof.

The zinc hydroxychloride used in this invention is a water-insolublezinc compound which has the formula Zn₅ (OH)₈ Cl₂.H₂ O. Though forpurposes of this invention the non-hydrated zinc compound, Zn₅ (OH)₈Cl₂, is equivalent to this hydrated form, the invention is exemplifiedherein by the use of the hydrated form.

The zinc hydroxychloride can be admixed with the alkali metal silicatesolution in any practical form, such as a dry powder or, preferably, asan aqueous paste (the preparation of which is described in the examplehereinbelow). The amount of zinc hydroxychloride added to the silicatesolution can vary and is sufficient to cause the applied coatingcomposition to become substantially water insoluble in the course ofdrying the applied coating composition. (Minor water-soluble components,such as NaCl, in the dried coating may be formed and leached from thecoating by rainwater but these components are not detrimental to theutility of the coating nor do they diminish the essentiallywater-insoluble nature of the dried coating.) Generally, the amount ofzinc hydroxychloride used in preparing the coating composition will bethat sufficient to provide at least 0.5 mole per mole of the alkalimetal silicate used, and preferably will be about 0.8 to 1 mole per moleof silicate. Amounts of zinc hydroxychloride less than about said 0.5mole will generally impart algicidal activity to the dried coating butthe coating will not be as water insoluble as desired. On the otherhand, an amount of zinc hydroxychloride greater than about 1 mole doesnot provide any added benefit and may tend to cause chalking of thedried coating or otherwise affect its integrity.

The amount of water in the coating composition of this invention canvary and will be dependent on the particular mode of application chosento apply the coating composition to the substrate. Some modes ofapplication, such as spraying, may require a lower viscosity, i.e., agreater amount of water, than other modes, e,g., brushing. Generally,however, the coating composition will contain about 55 to 70 weightpercent, preferably about 60 to 65 weight percent, water, the balancebeing silicate and zinc hydroxychloride solids.

The coating composition dries to a white coating, the zinchydroxychloride functioning to provide the white color. (The whiteningeffect of the zinc hydroxychloride is greater than that obtained bymixing the silicate component with an equivalent amount of zinc oxidepigment.) If other colors for the coating of this invention are desired,various pigments, e.g., cobalt aluminate for blue, chromium oxide forgreen, titanium dioxide for additional whiteness, and ferric oxide forred, as well as other well-known pigments, can be added to the silicateor zinc components prior to mixing or to the composition after mixingthe two essential components.

The coating composition of this invention is fairly stable and can bestored for several weeks but because there is a gradual gelling, whichmay hamper application of the composition to a substrate, and thus makethe coating operation less efficient, it is preferred to mix thesilicate solution and zinc hydroxychloride shortly before the resultingcoating composition is applied to the substrate. Thus, the coatingcomposition can be stored, shipped and marketed as a two-part ortwo-package system.

Any substrate which is normally susceptible to algal infestation, andthe consequent discoloration produced by algae growth thereon, can becoated with the coating composition of this invention. Normally suchsubstrates will be relatively rigid, inorganic, exterior building ordwelling structures found in the coastal areas of Florida, and otherregions of high temperature and humidity, which are exposed to theair-borne microorganisms. Generally, these substrates will be made of orcontain silicates, such as Portland cement concrete, fired clay, andmineral granules used in roofing or siding. The most prevalent andnoticeable substrate where this invention finds particular applicabilityis concrete roofing tiles which, as mentioned hereinbefore, areparticularly susceptible to algal infestation and develop an unsightlydark appearance because of such infestation.

The coating composition of this invention can be applied to a new, orrelatively new, or old and weathered substrate, for example the concreteroofing tiles of a house. The coating composition can be applied to asubstrate which is relatively old and infested though before doing so itwill be preferable to clean the substrate of dirt or algal infestationto insure good adhesion of the coating to the substrate. A particularlydesirable cleaning technique will be the aforementioned sodiumhypochlorite cleaning with subsequent washing to remove foreign matterfrom the substrate.

The applied coating composition is allowed to dry at ambienttemperatures (e.g., 60° to 100° F.) for several hours, or preferablyovernight, and a second application can be applied thereafter ifdesired. If rain is anticipated, the application of the coatingcomposition should be postponed until after the rain and after thesubstrate is dry; if a coated substrate is rained on before the coatingis sufficiently dry, the sodium silicate may still be significantlywater soluble and the rain may well carry away some of the silicatebefore its insolubilization, necessitating a reapplication of thecoating. Where the coating composition is to be applied to shingles orother roofs having granules embedded in asphalt, it may be desirable tofirst prime the surface to insure good adhesion of the coating, orsuitable wetting agents may be added to the coating composition toinsure good adhesion. It is also within the scope of this invention tocoat the building materials in the course of their manufacture before astructure is built; for example, concrete roofing tiles can be coated inaccordance with this invention in the manufacturing plant where thetiles are made.

The amount of coating composition applied to the substrate can vary and,functionally stated, the amount applied to the substrate will be thatsufficient to render the substrate resistant to algal infestation.Generally, about 1 to 20 pounds, preferably 5 to 15 pounds, of coatingcomposition per 100 square feet of substrate will be sufficient toprovide adequate coverage for that purpose, although lighter and heaviercoatings can be applied, the heavier coatings preferably being appliedin several steps with drying of each coating before application of thenext. The dry coating will have at least about 0.2 pound of chemicallycombined zinc per 100 square feet of surface area; generally, the amountof said zinc will be 0.8 to 2.5 pounds per 100 square feet.

In addition to rendering the substrate resistant to algal infestationand insuring a long-lasting pleasing appearance, the coating of thisinvention provides significant fire-proofing, particularly where thesubstrate is a mineral-surfaced asphalt roofing. This is due to thenonflammable nature of the coating, it being inorganic, and thecapability of the coating to form a crust which restricts burning andflowing of asphalt when the same is subjected to the heat of a fire. Thecoating of this invention is also relatively permeable or "breathable";thus, it is not a barrier and water or water vapor may escape from orpass through the coated substrate.

Objects and advantages of this invention are further illustrated in thefollowing example but the various materials and amounts thereof, andother details in the example, should not be construed to unduly limitthe scope of this invention. In the example, all parts given are byweight unless otherwise noted.

EXAMPLE I

A water solution of sodium silicate was made by diluting 116.4 parts ofa commercially available sodium silicate solution ("N" brand, having aNa₂ O:SiO₂ weight ratio of 1:3.22 and a specific gravity of 1.39) with43.5 parts water, this diluted solution hereinafter referred to as "PartA."

A 20 wt. % aqueous solution of zinc chloride was made by dissolving 24.5parts anhydrous zinc chloride in about 100 parts water. To thissolution, 52 parts of pigment grade zinc oxide was added with vigorousstirring. When the resulting slurry was smooth and free of lumps, it waspoured into polyethylene molds, which were then covered to preventevaporation. The slurry hardened in about 5 to 10 min. and was allowedto age for at least 24 hrs. to substantially complete the reaction ofthe zinc oxide and zinc chloride to zinc hydroxychloride. (As analternative to aging, the poured slurry can be heated and kept atelevated temperature, e.g. 150° F., for about 30 min.). The resultingsolid zinc hydroxychloride was removed from the molds, broken intopieces about 1 inch in size, and milled 4 hrs. in a ball mill with aporcelain grinding medium using about 11/4 parts water for each partzinc hydroxychloride. The resulting slurry was neutralized with a 20 wt.% sodium hydroxide solution until slightly alkaline to phenolphthaleinto neutralize any unreacted zinc chloride, and the neutralized slurrywas filtered to form a stiff paste containing about 42 wt. % solids,said paste hereinafter referred to as "Part B."

About 1.2 parts of Part A were stirred into 5.82 parts of Part B, andthe mixture stirred for about 2 min. Then 4 further parts of Part A wereadded slowly to the mixture with stirring until the mixture was smoothand free of lumps. (A well-dispersed paste of a tinting pigment can beadded if desired at this point or such pigment can be added during theball-milling of Part B.) In this coating composition, there are about0.82 mole of zinc hydroxychloride per mole of silicate and about 64 wt.% water, the balance being the silicate and zinc hydroxychloride solids.

A thin coating of the coating composition was applied to a cleanedconcrete tile roof in southeastern Florida using standard paint brushesand rollers. The coating was allowed to dry overnight and a similarsecond coat was applied over the first. The total coating composition(wet) amounted to about 11 pounds per 100 sq. ft. of roof surface area.The total coating contained about 1.75 pounds of chemically combinedzinc per 100 sq. ft. of surface area. The coated roof remained white andwas free of algae when examined 12 months later. Other concrete tileroofs in the area which were not so coated showed definite algaediscoloration 6 months after cleaning and painting with conventionalpaints.

EXAMPLE II

Samples of Portland cement concrete tiles, concrete blocks, andmineral-surfaced asphalt shingles were coated as described in Example I.The coated samples were tested for resistance to the growth of algae byattaching them to a test rack positioned at a 45° angle and facing northnear Houston, Texas. In order to intensify the exposure to algalinfection, an algae infested seed shingle, removed from an infestedroof, was placed on the rack directly above the coated test samples, anda further algae infested seed shingle was placed on the rack directlybelow the test samples. This arrangement permitted rainwater to run fromthe surface of the top seed shingle down across the surface of the testsamples and thence down across the bottom seed shingle. Upon examiningthe rack after a period of 15 months of exposure, the coated testsamples were found to be as white as they originally appeared and showedno visual evidence of algae growth; the lowermost seed shingles werefound to have significantly less algae growth than the top seed shingleand, in fact, considerably less growth than it originally had at theoutset of the test period. This test demonstrated the resistance of thecoated substrates of this invention to algal infestation and thealgicidal nature of the coated substrates.

Various modifications and alterations of this invention will be obviousto one skilled in the art without departing from the spirit and scope ofthis invention, and it should be understood that this embodiment is notto be limited to the illustrative embodiments set forth herein.

What is claimed is:
 1. An inorganic coating composition comprising about 55 to 70 weight percent water, a water-soluble alkali metal silicate, and zinc hydroxychloride of the formula Zn₅ (OH)₈ Cl₂, in an amount of at least 0.5 mole for each mole of said alkali metal silicate, said composition upon being applied as a coating and dried at ambient temperatures of about 60° to 100° F. forming a hard, durable, water-insoluble, water-permeable, weather-resistant, inorganic, algicidal coating.
 2. An inorganic coating composition comprising about 55 to 70 weight percent water, a water-soluble sodium silicate having an Na₂ O:SiO₂ weight ratio of 1:2.75 to 1:3.75 and zinc hydroxychloride of the formula Zn₅ (OH)₈ Cl₂.H₂ O in an amount of about 0.5 to 1.0 mole for each mole of said sodium silicate, said composition upon being applied as a coating and dried at ambient temperature of 60° to 100° F. forming a hard, durable, water-insoluble, water-permeable, weather-resistant, inorganic, algicidal coating.
 3. A coating composition containing about 55 to 70 weight percent water, sodium silicate and about 0.8 to 1 mole of zinc hydroxychloride per mole of sodium silicate, said composition being formed by mixing an aqueous inorganic solution containing 25 to 35 weight percent water-soluble sodium silicate, having an Na₂ O:SiO₂ weight ratio of 1:2.75 to 1:3.75, with an aqueous paste comprising 35 to 55 weight percent zinc hydroxychloride, of the formula Zn₅ (OH)₈ Cl₂.H₂ O, said composition upon being applied as a coating and dried at ambient temperature of 60° to 100° F. forming a hard, durable, water-insoluble, water-permeable, weather-resistant, inorganic, algicidal coating.
 4. An inorganic coating composition comprising about 55 to 70 weight percent water, a water-soluble sodium silicate having an NaO₂ :SiO₂ weight ratio of 1:3.22 and zinc hydroxychloride of the formula Zn₅ (OH)₈ Cl₂.H₂ O in an amount of about 0.5 to 1.0 mole for each mole of said sodium silicate, said composition upon being applied as a coating and dried at an ambient temperature of 60° to 100° F. forming a hard, durable, water-insoluble, water-permeable, weather-resistant, inorganic, algicidal coating. 